Every 780 days (a little over 2 years) Earth catches up with Mars in its orbit. We call that moment “opposition,” when Mars is exactly opposite the Sun in the sky. At that time, Mars is also as close to us as it gets, so that’s the best time to observe it. Likewise with all the other outer planets: They’re best observed at opposition.

The time between oppositions is easy to discern, and astronomers have been doing so since long before the invention of the telescope. Knowing that number, and knowing how long Earth takes to go around the Sun (365.25 days), they could calculate the orbital period of the outer planets. At our April 17th meeting, Bob Andersen will talk about planetary conjunctions and how they relate to orbital periods. His presentation will dig into some interesting concepts like infinite series and Zeno’s paradox, and Bob will also show us how to determine conjunction times if we know the length of a planet’s year. Come learn some interesting things about the mechanics of our solar system!